Car will not crank or start plzzz help

So one morning I go out & my car cranked
but wouldn’t start so I thought okay dead
battery. I had it jumped and it started up
good when I got out of school, then the next
morning I go out & it’s completely gone no
crank or anything just dead so we thought
okay it’s time for a new battery because the
one we had was old. It started right up with
the new battery I went out the next morning
and it was completely dead again so we
thought it was the battery terminals because
they were really rusty. We had those replaced
in the car wouldn’t start it was dead wouldn’t
even crank. So then we finally thought that it
was our alternator we got a brand new one
replaced and put it in & it started right up. I
came out from work and it was completely
dead again no crank no nothing I really don’t
know what’s going on is someone could help